In the production of polyvinyl chloride resins by suspension polymerization, the monomer or monomer mixture is finely distributed in the aqueous phase by mechanical agitation, stabilized by addition of protective colloids or suspension stabilizers, and polymerized in the presence of free radical catalysts.
Suitable suspension stabilizers include water-soluble, natural polymeric materials, such as gelatin, starch, and the like; water-soluble cellulose derivatives, such as alkyl celluloses, hydroxyalkylcelluloses, and the like; water-soluble, synthetic products, such as partially saponified polyvinyl acetates, copolymers of maleic anhydride and vinyl acetate, and also various combinations of the aforementioned materials.
Polyvinyl chloride resins produced using the aforementioned suspension stabilizers yield, in soft processing, finished components having a large number of so-called gels or fisheyes. This is due to their unsatisfactory absorption of plasticizer. Furthermore, residual monomer can be removed only with great expense and difficulty due to their low granular porosity and compact granular structure. In order to overcome these drawbacks, it has been suggested (DOS Nos. 2,528,950 and 2,629,880) to use partially saponified polyvinyl acetates having degrees of hydrolysis of 20-65 mole % and degrees of polymerization (number average) of 200-3,000 as cosuspension stabilizers in combination with, preferably, cellulose derivatives and partially saponified polyvinyl acetates having degrees of hydrolysis of more than 65 mole %, in order to improve plasticizer absorption, granular porosity and reduce gel content.
Although porosity-enhancing agents improve the properties of the resins for soft processing, these resins still exhibit an unsatisfactorily high gel content due to the deficient properties of the primary suspension stabilizers. This is especially true if the polyvinyl chloride resin combined with plasticizers is processed at high processing speeds. In addition, such products still have too low a porosity for ready performance of vinyl chloride degasification for maximum removal of unreacted monomer.
In polyvinyl chloride resins manufactured by conventional methods using the usual suspension stabilizers, an interaction exists between porosity and bulk density. An increase in porosity, for the purpose of satisfactory monomer removal, causes a great reduction in bulk density. A reduction in bulk density causes a decrease in the output of extruded polyvinyl chloride resin.